Re: [PATCH 6/6] ioctl eofblocks: require non-privileged users to specify uid/gid match

Brian Foster
2013-06-28T21:42:30Z

On 06/28/2013 04:28 PM, Dwight Engen wrote:
> On Fri, 28 Jun 2013 14:50:24 -0400
> Brian Foster <bfoster@redhat.com> wrote:
>
>> On 06/28/2013 11:11 AM, Dwight Engen wrote:
>>> Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
>>> ---
...
>>
>> In thinking more about the other aspect of group management (and I
>> admit I'm still waffling about this), it seems like we could go in a
>> couple directions:
>>
>> - Now that we have a separate internal only eofblocks control, be more
>> flexible and provide an internal only flag (valid only for UID/GID
>> scans) to instruct the scan to do specific file permission checking
>> against the inodes. This would be set by xfs_file_ioctl() and do the
>> write permission enforcement for userspace originated scans. This
>> would also allow the future EDQUOT work to leave out said flag and do
>> a group wide scan regardless of the specific permissions of the
>> calling context (i.e., when the system decides all inodes under a
>> group quota must be trimmed).
>
> I haven't seen your EDQUOT change, but your description made me wonder:
> Are you going to kick off a scan for the type of quota exhausted?
> Otherwise could a user abuse this by overrunning his user quota in
> order to cause group inodes (that he may not have write to) to be
> reclaimed?
>

Yes, you could describe it that way. The current behavior is a global
inode flush followed by an ENOSPC/EDQUOT error, so I'm not sure we're
exposing much by attempting an eofblocks scan before running out of
space. It's a fairly minor optimization in the failure path.

Another way to look at it might be that the users/inodes have never
really reserved the extra space that's being trimmed here, so the
filesystem has every right to take it away if it deems it better to put
to use elsewhere (i.e., to put off an ENOSPC related failure).

> At any rate, yeah the only way I see to get the permissions checks
> right is to set a flag up in ioctl because the checks need to be per
> inode. I think you would like to avoid having the checks that low in
> xfs_icache, but I don't see a way around that.
>

That's the reasoning behind the extra internal only flag. It's not
having the check that I'm against so much as the idea that this layer of
code should be unconditionally bound by the current userspace context.
But it might be reasonable to add control flags to effectively make that
so (i.e., XFS_EOF_FLAGS_ENFORCE_PERMS).

>> The downsides here are the behavior might be a bit unclear and we'd
>> have to fork off the flags bits in a manner that's clear and
>> maintainable. I suppose one could also argue this is overkill for
>> somewhat of a secondary operation.
>>
>> - Go the other direction, be less flexible and simply not allow
>> !capable(CAP_SYS_ADMIN) group scans just as we started doing for
>> project quotas. This is obviously very simple, but then we disallow
>> regular users from trimming groups of inodes they should full well
>> have permission to trim.
>
> What about uid == self scans? Would we not allow those as well? The
> check may be simpler than a group check but still would need to be per
> inode, and thus still need the flags of option 1.
>

I'm not following. Aren't we already enforcing this appropriately with
your current patch? My comments were intended to be taken as in addition
to this patch. i.e., with regard to your comments in the commit log here:

http://oss.sgi.com/archives/xfs/2013-06/msg00785.html

>> I think I'm leaning towards the former approach if it can be
>> implemented cleanly. Thoughts or ideas?
>
> Well the former is certainly more functional and allows the trimming to
> be under the users control. How useful is that though once it happens
> automatically with your EDQUOT changes? If we only allowed the ioctl to
> trigger global scans, we wouldn't need a conversion function nor
> separate structure, but I don't know the use cases for uid/gid specific
> scans from userspace so its hard for me to judge the tradeoffs.
>

To be honest, there aren't any real users of the eofblocks command from
userspace that I'm aware of at the moment. I added it originally for a
poc quota implementation I was working on for gluster, but the primary
use case for the scanning mechanism is to allow background clean up of
files such that post-eof speculative preallocation doesn't hang around
for too long.

> Maybe this permission stuff should be a separate change since it isn't
> really related to user namespace stuff? I just happened to be in the
> vicinity and am happy to help :)
>

Sounds reasonable to me. :)

Brian